The inner capsid protein of rotavirus, designated VP6 and having a molecular weight of approximately 45 kd, has been disclosed as an immunogen for use in vaccines to prevent rotaviral infection and also, more importantly, as a carrier for an immunogenic complex generally. U.S. Pat. No. 5,071,651 filed 2 Mar. 1990 which is a file-wrapper continuation of U.S. Ser. No. 092,120 filed 2 Sep. 1987, describes in detail the advantageous properties of the VP6 rotaviral protein as an immunological carrier. The contents of the specification are incorporated herein by reference.
As described in the above specification, VP6 is the most abundant structural protein in rotavirus and has an approximate molecular weight of 45 kd. The gene encoding this protein has been cloned and the protein has been produced by recombinant methods (Estes et al., Nucleic Acids Res (1984) 12:1875-1887; Estes et al., J Virol (1987) 61:1488-1494). The complete amino acid sequences as well as the sequences of the VP6-encoding DNA is known for at least nine strains of rotavirus.
The above-referenced application describes methods for the recombinant production of the VP6 protein and describes its structural characteristics. Of interest herein, VP6 protein is known to aggregate into tubular particles under conditions from about pH 5-pH 9, which particles are moderately stable to changes in temperature and ionic strength. Spherical particles resembling single-shelled virus can be formed at about pH 4 and are now known to be approximately 2.times.10.sup.-22 meters.sup.3 in volume with a diameter of about 70 nm. The surface of the sphere appears to be perforated by 132 channels which are 40-60 angstroms in diameter (Yaeger, et al. J Cell Biol (1990) 110:2133-2144). A copending application Ser. No. 07/650,054 filed on even date herewith, describes encapsulation of various biological effector molecules in these spheres. In addition, the VP6 protein can be made to form sheets composed of a small-holed lattice in samples where the solutions have been shifted from about pH 6 to about pH 4.
As described in the above-referenced U.S. Pat. No. 5,071,651, VP6 protein acts as a carrier useful in immunogenic complexes to provoke immunogenic responses to haptens or antigens coupled to the surface of the carrier. The hapten or antigen may be coupled to the carrier using standard conventional coupling techniques, including coupling of polypeptide or protein haptens or antigens to the carrier by synthesis of a fusion protein combining the VP6 with the antigen. Other methods include, for example, coupling of carbohydrates using reductive amination techniques and coupling of carbohydrates or other proteins using linkers such as those available from Pierce Chemical Company, Rockford, Ill.. Unique to VP6, however, is its ability to interact with a "binding peptide" through protein-protein interaction, provided the binding peptide has a region of appropriate sequence, as explained in the referenced application. The binding peptide can be used to mediate binding of antigen or hapten to the VP6 carrier by coupling the antigen or hapten to the binding peptide and then binding to VP6 through protein-protein interaction.
While the use of VP6 as an immunological carrier is clearly disclosed in the above-referenced application, there is no suggestion or realization that VP6 particles are useful to mediate diagnosis by virtue of their ability, described herein, to home specifically to macrophages and monocytes. In addition, there is no suggestion that the VP6 carrier, by virtue of its ease of interaction with specific targeting agents can be used to effect targeted drug delivery or localization of label to additional targets.